I Know What You Did Last Summer

"I Know What You Did Last Summer" is a narrative duet that explores the emotional turmoil of a romantic relationship on the brink of collapse due to infidelity. Sung from the perspectives of both the person who was unfaithful (Camila Cabello) and the person who was cheated on (Shawn Mendes), the song captures the raw and painful conversation that occurs when a secret is exposed. Cabello described it as a "conversation between two people in a relationship where it's dying, but nobody wants to admit that it's dying." The lyrics convey a sense of paranoia, guilt, and heartbreak. Cabello's verses reveal her inner torment and the knowledge that her partner knows her secret, while Mendes's verses express his pain and the struggle of seeing the person he loves changed by their actions. The core theme is the agonizing moment of confrontation, where both parties are aware of the betrayal, and the air is thick with unspoken truths and desperate emotions.

The song unfolds as a tense and emotional conversation between two lovers, grappling with the aftermath of infidelity. From the woman's perspective, there's a heavy burden of guilt. She holds onto "dirty secrets," and this knowledge is tormenting her. She's aware that her partner knows about her transgression, symbolized by the line, "He knows another's hands have touched my skin." She avoids his questions, refusing to disclose where she has been, yet she is acutely aware that her secrecy and actions are destroying him and their relationship. The internal conflict is immense; she feels him slipping away and questions if he's only clinging to the memory of who she used to be and the words she used to say. The pictures on her phone serve as evidence of her betrayal, a constant reminder that she is not returning to the relationship in the same way.

From the man's perspective, there is a painful certainty of betrayal. He confronts her directly, demanding to know where she's been, although he already knows the answer. The phrase "I know what you did last summer" is a direct accusation, and he asks her to lie to him, perhaps to soften the blow or because he isn't ready to face the unvarnished truth. He observes the change in her, noting that when she looks at him, her eyes "don't seem as bright no more." He reminisces about a time when he was certain of her love, a promise he made to her that now feels hollow. He feels her slipping away and sees the evidence of her distance in the pictures on her phone. He is caught in a painful limbo, unable to let her go but also unable to keep her close because of the broken trust.

The bridge of the song is a chaotic and overlapping exchange of their conflicting emotions and desperate pleas. They both express the torment of their situation: the inability to let go versus the impossibility of staying close. She pleads that she "didn't mean it," a cry for forgiveness and understanding. He, in turn, asks her to be honest and mean what she says. This section captures the heart of their struggle—a messy, painful negotiation of hurt, regret, and a flickering hope for reconciliation that seems just out of reach. The repetition of "I know, I know, I know" mimics a torturous, obsessive thought loop, emphasizing the inescapable reality of the betrayal that hangs between them.

The creation of "I Know What You Did Last Summer" was remarkably spontaneous. The song was written backstage during one of Taylor Swift's The 1989 World Tour shows in the summer of 2015, where Shawn Mendes was an opening act and Camila Cabello, then a member of Fifth Harmony, was a guest. In a chaotic dressing room with about 50 to 100 people, Mendes started jamming on his guitar. Cabello was drawn to the melody and they began improvising and writing together. They crafted the pre-chorus and chorus in about 30 minutes. The initial idea was to create a conversational song between a boy and a girl. Feeling they had something special, they went into a studio the next day and completed the song during an 11-hour session. The bridge of the song was notably improvised in the studio; the producers had them in separate booths with the lights off, and they sang their emotional back-and-forth, with those raw vocals being used in the final track. The song was co-written by Mendes and Cabello along with producers Ido Zmishlany and Noel Zancanella. Bill Withers also received a songwriting credit due to the interpolation of the "I know, I know, I know" refrain from his song "Ain't No Sunshine".

The song is set in the key of A minor with a tempo of approximately 114 beats per minute. The rhythm is driven by a persistent and somewhat tense acoustic guitar strumming pattern, which can be challenging due to its speed and use of muting to create a percussive effect. This rhythmic drive propels the song forward, reflecting the urgency and anxiety of the lyrical confrontation. The rhyme scheme is relatively straightforward, often using simple end rhymes within the verses and chorus to maintain a pop structure and catchiness. However, the emotional core is less about complex rhyme and more about the interplay between the two vocal rhythms. The lyrical rhythm is conversational, with phrases that feel like natural speech, especially in the overlapping bridge. This creates a powerful contrast with the steady musical rhythm, highlighting the friction and disconnect between the two characters.

Musically, the song is built around an acoustic guitar riff that provides a sense of intimacy and urgency. It employs a call-and-response structure typical of a duet, creating a conversational and confrontational dynamic. The vocal deliveries are emotionally charged, escalating in intensity to match the narrative. The bridge is a particularly notable section where the vocals become layered and almost frantic, a technique called homophonic phrasing, where different overlapping lines compete for attention, mirroring the characters' chaotic emotional state. Lyrically, the song relies on repetition to create emphasis and convey a sense of obsession and inescapable thought. The repeated phrase "I know what you did last summer" serves as a central hook and accusation, while the mantra-like repetition of "I know, I know, I know" echoes the cyclical and torturous nature of the knowledge of betrayal. A study has pointed out the use of various repetition types like alliteration, assonance, and consonance throughout Mendes's work, which contribute to the musicality and emotional weight of the lyrics.

Released on November 18, 2015, "I Know What You Did Last Summer" served as Camila Cabello's debut single as a solo artist outside of Fifth Harmony. The song was a commercial success, reaching the top 20 on both the US Billboard Hot 100 and the Canadian Hot 100. It debuted at number 97 on the Billboard Hot 100 and eventually peaked at number 20. The song's immediate popularity was evident as it climbed to number 3 on the US iTunes sales chart shortly after its release. The track was the lead single for the reissue of Mendes's debut album, titled "Handwritten (Revisited)". Mendes and Cabello performed the song on several major television shows, including *The Tonight Show Starring Jimmy Fallon*, *The Late Late Show with James Corden*, and at the 2016 People's Choice Awards, which boosted its visibility. The song marked the first majorly successful collaboration between the two artists, who would later team up again for the global hit "Señorita" in 2019. The title, while claimed to be a coincidence, is the same as the 1997 slasher film, which added a layer of pop culture reference, though the song's theme is about cheating, not murder.

The music video for the song heavily uses weather as a metaphor for the emotional state of the relationship. The dark, desolate landscape represents the barren and broken state of their connection. As Mendes and Cabello walk towards each other on treadmills, they never get any closer, symbolizing their emotional distance and the inability to bridge the gap created by the betrayal. The progression of the weather from a dust storm to a snowstorm and finally a torrential downpour mirrors the escalating intensity of their argument and emotional pain. The storm is an external manifestation of their internal, tumultuous conflict. The "pictures on her phone" are a tangible symbol of the infidelity, concrete evidence that haunts him and confirms his fears. Her eyes, which "don't seem as bright no more," symbolize the loss of innocence and love in their relationship from his perspective.

The most significant recurring phrase is the title itself, "I know what you did last summer." It functions as the central accusation and the anchor of the song's narrative. Its repetition throughout the chorus drills the theme of betrayal into the listener's mind. Another key recurring motif is the line "Can't seem to let you go, can't seem to keep you close." This phrase, repeated by both artists, encapsulates the central conflict of the relationship: the inability to reconcile and the equal inability to part ways. This push-and-pull dynamic is the emotional crux of the song. The repetitive chant of "I know, I know, I know, I know" is a direct interpolation of Bill Withers' "Ain't No Sunshine" and serves as a motif of obsessive, painful knowledge that cannot be escaped. This repetition expresses an "emotional claustrophobia" as the thought bounces back and forth between them.