Warning: Official policy states that no spoiler tags should exist on recap pages. Read at your own risk.
In the aftermath of Gabriel's near-exposure on Heroes Day, he becomes concerned that his time is limited, and turns to extreme measures. He deliberately puts excessive pressure on Adrien, in hopes that he will become an especially powerful akuma due to his emotional connection to his mother.
Marinette notices Adrien's stress, and resolves to help him. Her investigations lead her to discover his crush on Ladybug, which startles her, but she holds to her plan of merely being a supportive friend, instead of deliberately pursuing him in costume. As Ladybug, she protects him from incoming butterflies and gives him contact details for a disposable phone and email address; as Marinette, she talks with him at school and lets him unburden himself. Gabriel is frustrated by Adrien's mood improving, but notices how closely he seems to be connected to Ladybug, and waits for an opportunity to make use of it.
One of Ladybug's visits then turns romantic, when she finds out that he composed a song for her, and shortly afterward, when talking at school, Marinette admits her feelings for him. Adrien feels attracted to both identities and isn't sure how to react, but tries to remain honest and open about continuing to see both of them. Gabriel becomes aware of the apparent love triangle and finds it hilarious, believing that it will eventually break and give him a prime akuma target when one of the three is disappointed.
Adrien eventually chooses Marinette, after learning of her selflessness regarding the scarf she gave him. Shortly afterward, however, he puts several clues together and realises that they're the same girl. He hints to her that he knows, but doesn't force her to admit it.
He also approaches Nino, intending to hand over the Cat Miraculous, since it's dangerous for Ladybug and Chat Noir to know each other's identities. However, this only results in further reveals, as Nino realises both his and Marinette's identities, and gives away himself and Alya. Plagg is exasperated by the attempt, but persuades Adrien to keep the ring and report to the Guardian. Master Fu is concerned about the reveals, but supportive of Adrien's relationship with Marinette.
Marinette then comes to dinner at the Agreste mansion as Adrien's girlfriend, at Gabriel's invitation, and their kwamis explore — encountering Nooroo and confirming their suspicions that Gabriel is Hawkmoth. Plagg evacuates Adrien, while Tikki informs Marinette and has her transform to confront Gabriel. However, Gabriel persuades her to hold off on an immediate fight, expressing a desire to negotiate, and tells her about Emilie and his plans for the Wish. He intends to create a powerful healing akuma, drawing on the emotional bond between Adrien and his mother; strengthen Emilie as much as possible; then use the Wish to make up the difference and end her coma, at the cost of someone else's health, who could then also be healed by the akuma. Marinette is uncertain about the situation and about whether his plan can work, but decides to retreat and consult with the Guardian rather than risk fighting him alone.
Master Fu is startled by the news, and uncertain about the feasibility of the plan, but consults with the kwamis and decides to accept the risk instead of trying the risks of a full-scale war. Marinette and Adrien return to the mansion and the plan proceeds, with Adrien becoming "Lifeline" and Marinette wearing the Cat and Ladybug Miraculouses. Adrien is able to restore most of his mother's health, and then Marinette uses the Wish to push her further at the cost of her own health.
However, Marinette then slumps, worn out, and Gabriel orders Nathalie to steal the two Miraculouses, enraging Adrien. Nathalie is ambushed and pinned by Carapace, and discovers that the Miraculous "dropped" by Marinette were a Fox illusion created by Master Fu.
Adrien's fury then interferes with his ability to heal Emilie and Marinette, prompting Gabriel to step in. He instructs Nooroo to convert Lifeline from a light akuma to a dark one, changing Adrien's powers from simple Healing Hands to a Vampiric Draining, then uses the new powers to vacuum out Gabriel's own life force and supercharge Emilie's healing. Emilie is revived, while Gabriel collapses into a coma.
In the aftermath, it turns out that Gabriel is now present as a voice in the back of Emilie's head, and guides her in keeping the company afloat. Marinette tells her parents about her Miraculous, and they are given the Butterfly Miraculous, which they take turns using to empower ordinary civilians into heroes. Adrien's bodyguard is given the Monkey Miraculous and goes into crime-fighting.
