In the episode, after the events of "Detained", the group fell apart, unable to recover from the truths they learned about each others, when the king of the Shadow Kin Corakinus (Paul Marc Davis), resolute to take April's (Sophie Hopkins) heart and the Cabinet of Souls, attacks and threatens to kill all their loved ones until he has what he desires.

When the Shadow Kin invade Earth and take over the streets, Charlie is left with no choice but to use the Cabinet of Souls, which is expected will also kill April and himself.

Elsewhere, Ames returns to the Governors where she is judged unfit to continue serving them or witness "the arrival" for having allowed the Cabinet to be used, and is murdered by a Weeping Angel.

The development of the characters, and performances, especially from Greg Austin, Katherine Kelly, and Jordan Renzo were praised, but many reviewers felt that the episode was too fast-paced, and did not feel like a satisfying conclusion for the season's storylines.

Entertainment Weekly's Nivea Serreao and Kelly Connolly both gave a positive review, praising the episode's stakes, twists and characters, especially Tanya.

"[6] Flickering Myth called "The Lost" "a disappointing finale to Class – one that, ultimately, is unable to deliver upon the promise of the series as a whole, and leaves the show’s future looking rather uncertain."

Regarding the ending, they stated that it "confirms the growing suspicion that the Governors were a far more interesting antagonistic force than the Shadowkin could ever have been – something we hadn’t seen before, something new, and by extension something exciting.

[7] Cult Box gave a very positive review, stating that Class "has beaten its own path rather than relying on elements from the main show.

However, with the creative vision that Patrick Ness has demonstrated throughout, we have faith that he will put a new spin on the killer statues and we look forward to seeing where he takes them and Coal Hill Academy in the future.

[9] Mym Buzz gave a positive review, considering that "'The Lost' offered up an incident-packed, revelation-rammed, fast-paced, visually spectacular season finale – and there were lots of really fun moments, exquisite character exchanges and big dramatic gestures to savour".