An Aperture scientist, who'd barely escaped the neurotoxin incident and now suffers amnesia because of it, finds themselves in a dilapidated wing of Aperture, located at the bottom of the ocean. As they attempt to escape, they encounter a personality core, who tuns out a bit more familiar than the protagonist expected. Portal Psi is a Portal mod focused on visual and literary storytelling.

As the protagonist attempts to escape, they learn of their connection to the personality core. This, however, leads them to drift apart, until at the end, they face the ultimate decision that affects both them and other scientists of the once-glorious Aperture Science.

Clean for testchamber contrasted with grungy industrial for backstage areas. As the story goes on, the styles mix, and the chambers show real dilapidated and abandoned state of the facility.

A monoportal-centered testing course, interweaved with creative BTS challenges, where the protagonist and the sidekick fight against the hostile complex.