Katniss wakes in District 13 after the Quarter Quell and learns that District 12 has been firebombed. Peeta is being held by the Capitol and appears in Snow's broadcasts urging a ceasefire. To the rebels this looks like betrayal. In fact, he is being coerced and tortured.
Katniss agrees to be the Mockingjay on the condition that Peeta and the other captured victors will be pardoned. Her staged studio propos fail. In District 8, the Capitol bombs a hospital full of wounded people, and her furious, unscripted speech ('if we burn, you burn with us') becomes the rallying cry that ignites the districts. Her performance of 'The Hanging Tree' becomes an anthem, and rebels storm and destroy a Capitol dam, cutting off power.
During a live broadcast, a battered Peeta warns District 13 of an imminent attack and is visibly beaten on air. The warning saves District 13 from the bombing that follows. While Katniss draws Snow's attention with a propo, a rescue team including Gale brings Peeta and other captives back.
The twist comes when Katniss greets Peeta: he attacks her and nearly strangles her before Boggs knocks him out. He has been 'hijacked'. Tracker-jacker venom was used to overwrite his fond memories of Katniss with fear, turning him into a weapon against her. Snow let the rescue succeed on purpose. Looking back, Peeta's broadcasts, and the ease with which the rescue team got him out, were part of Snow's psychological war. The victory is really a trap that turns Katniss's greatest love into her greatest threat, and it sets up Part 2.