Plot summary
Mack, struggling with the departures of Morse and Hunter, takes time off to be with his brother Ruben. When an online hate group called the Watchdogs uses Howard Stark -developed nitramene bombs to perform a terrorist attack on an ATCU facility, Coulson suspects that former S.H.I.E.L.D. agent Felix Blake —who had been committed to the advancement of nitramene technology and has been missing since having his back broken by Deathlok —has taken over the group. Ruben becomes disillusioned with Mack when he leaves to investigate a Watchdog base and accidentally reveals his S.H.I.E.L.D. agent status, but Mack is able to improve their relationship when he fends off a group of the terrorists that attack the brothers.
Coulson takes Lincoln to search Blake's known safehouses and the two are confronted by Blake via hologram at one; he reveals that Hydra is backing him and the Watchdogs in their crusade to kill all Inhumans, and S.H.I.E.L.D. realizes that the attack was a cover for Hydra to steal something from the ATCU.
Credits
Adapted from Wikipedia contributors, Agents of S.H.I.E.L.D. season 3. HTML formatting and reference markers removed.