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The Tycho station Coup was an event where members of the Free Navy and defected OPA members attacked Tycho Station and stole the sole remaining Protomolecule samples from Johnson's supervision. During the assault, Fred Johnson was fatally wounded by Tycho chief engineer Sakai.
After reporter Monica Stuart had been kidnapped by unidentified individuals seemingly wanting her information on the location of the last Protomolecule sample being researched on under the supervision of Fred Johnson and Anderson Dawes and subsequently rescued by James Holden and Tycho security officer "Bull", Fred was made aware of the potential danger of a hostile takeover of the Protomolecule. This hunch was further supported by the kidnapping of Paolo Cortázar, the last remaining Protogen scientist, by an unknown Belter strike force on Ceres.
After following potential clues that might have revealed the group's identity ultimately turning out to led nowhere, Johnson discovered that an incoming Belter ship, the Zmeya, was supposed to pick up the container in which Stuart was held in. Suspecting them to think she is still in there, Holden and Johnson prepare a boarding party led by "Bull" hidden inside the container to board the Zmeya and simultaneously trap it inside Tycho Station's logistics port, hoping to uncover the true identity of the group.
However, the plan backfired dramatically, as the Zmeya pretended to suffer control issues, slowed down and surprisingly fired torpedoes at the container. Holden warned the party in time to escape the container, but the Zmeya destroyed the container and launched a retrofitted breaching pod at Johnson's private quarters in the habitation ring. Tycho chief engineer Sakai and OPA members ultimately betrayed Holden and Johnson, shooting him in the back and retreating to his quarters.
The docked pot released a vault robbery mech robot while Sakai and the defected staff killed the remaining loyal station security inside the quarters. The mech then searched and discovered the Protomolecule sample, in fact hidden under Johnson's bed before taking and returning it to the pod. Monica, still present as well, was forced to suit up by Sakai as they intended to forcefully take her again, before Holden intervened and gunfire ensued. After a fist fight between Monica and Sakai and Holden trying to interrupt the progress of the mech retrieving the sample, the pod then detached thus leaving a large hole to the vacuum of space open.
While Sakai was captured alive and most of the defected staff either captured as well or killed, the Zmeya recovered the pod and quickly fled the station. Later, Johnson finally succumbed to his fatal wounds. Sakai is then imprisoned and interrogated by Holden and Bull but fail to get any information. However, Holden tells them he has gotten the Zmeya's flight plan from Monica having seen it on Sakai's hand terminal. With this, Holden, Bull, Leveau, and a Tycho engineer get on the Rocinante to go after the ship. While they turn on the reactor, Holden gets a transmission message from Naomi, telling him the reactor had been set to detonate when fully activated by Sakai. They manage to shut down the reactor in time to stop the Roci and Tycho Station from being destroyed.