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The "Titan injection" is a fluid speculated to be synthesized from Titan spinal fluids, appeared at several plot points. When injected, it converts anyone (without the Power of the Titans) into a Pure Titan. The syringe was introduced in fragments of Eren's memory, where Grisha injects him to pass him the power of the Attack Titan. Later, we see Rod Reiss attempt employ it to have Historia regain the Founding Titan from Eren. It was also used to save Armin who was mortally wounded, by turning him into a titan, then gaining the Colossus Titan. We also learnt much later that Marley uses it on convicted Eldians, turning them into Pure Titans to roam Paradis Island.

The Cadet Corps train and educate cadets. Only the top 10 graduates can apply for Military Police after graduation. We follow Eren and his friends as they graduate from the 104th Cadet Corps. Unsurprisingly, the 3 Marley "spies", Annie, Reiner and Bertolt occupied the top 3 spots, likely due to their previous military trainings. Or it could also be due to their power of the Titan, as Eren ranked right below them.

In order to obtain the Founding Titan, Marley commenced the Paradis Island Operation, deploying a total of 3 Warriors, each with a specific role.
Annie, the Female Titan, transported the team to Wall Maria, while luring and gathering Pure Titans.
Bertolt, the Colossus Titan broke the outer wall, while Reiner, the Armored Titan breached in inner walls, spilling the Titans into Shiganshina.
This allowed them to infiltrate Paradis to attempt to steal the Founding Titan.

::: spoiler Correction
Um actually ... Marley deployed 4 Warriors. There is also Marcel, who was supposed to transport the team together with Annie, but he was eaten by a Titan in Paradis.
:::

After experiencing the trauma of his mother's death, Eren, together with Mikasa and Armin, enlists in the 104th Cadet Corps. They are joined with other cadets around their age, notably Annie, Reiner, Bertholdt, Ymir, Christa, Sasha, Jean, Marco and Conny. They are trained by Keith Sadies, who previously was the commander of the Scout Regiment.