Plot summary
Coulson, Fitz, and Reyes get trapped between dimensions and helplessly watch as Morrow kills several agents before escaping. Fitz overhears Mace arguing with Nadeer; he needs Simmons to understand Morrow's technology, but she has been taken to Nadeer's brother, Vijay, who she helps break out of his terrigen cocoon. In Simmons' absence, Radcliffe studies the technology and is presented with the Darkhold by May in her desperation to save Coulson. Radcliffe believes the book contains too much knowledge for a person to process, but Aida is able to read it, revealing her android nature to Coulson and May.
Aida constructs a portal through which Coulson and Fitz return. During this time, the Ghost Rider spirit had left Reyes and possessed Mack to avoid the other dimension, where it had been before. Reyes confronts the spirit, promising to serve it even after they defeat Morrow if it leaves Mack. It agrees and Reyes returns with it through the portal. Secretly, Aida begins experimenting with her new knowledge from the Darkhold, creating an artificial brain.
Credits
Adapted from Wikipedia contributors, Agents of S.H.I.E.L.D. season 4. HTML formatting and reference markers removed.