|For other uses, see Principia (disambiguation)|
At the Lighthouse, Jemma Simmons and Alphonso Mackenzie visit Yo-Yo Rodriguez. Phil Coulson informs Rodriguez that Leo Fitz intends to build prosthetics for her. As they leave Rodriguez, Fitz warns them about the Gravity Containment Device which is barely keeping the Fear Dimension from opening again. Therefore, they need to find more Gravitonium with the only lead they have on the matter: Cybertek.
Daisy Johnson investigates the Cybertek company and finds that several scientists who used to work on Gravitonium are presumed deceased. However, their death certificates were all signed by the same person, Murray Jacobson, so Johnson thinks that they are actually alive with new identities. Coulson, Mackenzie, Johnson and Melinda May decide to find Jacobson while Fitz and Deke Shaw remain at the Lighthouse, much to the former's dismay.
S.H.I.E.L.D. finds Jacobson, who happens to be Tony Caine, Mackenzie's old friend. They take him to the Zephyr One, where they explain the situation. Caine agrees to take Mackenzie to see Joseph Getty, Cybertek's chief scientist on Gravitonium, who reveals that Cybertek had a ship, the Principia, loaded with a cargo of Gravitonium, but it disappeared and was presumably sank, making the Gravitonium out of reach.
Aboard the Zephyr One, Coulson figures out that Johnson and May hope that Cybertek could also provide them with a way of saving him. He refuses to pursue such an objective and orders them to drop the matter, but Johnson and May are keen on trying anything.
Fitz unsuccessfully attempts to locate the Principia and ends up having an argument with Shaw. While Simmons comforts Fitz, Shaw leaves to find some food. He sees a recreation of his mother, which warns him that he has to leave before being destroyed by another recreation of a Kree Warrior. Shaw manages to destroy it and is found by Simmons. As they discuss about Fitz's behavior, Shaw figures out that the Principia was not sunk, but instead that the Gravitonium took it in the sky.
S.H.I.E.L.D. successfully finds the ship high above the ground and boards it. They find only a small sphere of Gravitonium, meaning that someone else took the rest. As they take whatever is left, they are attacked by Sleeper Mechs, the same androids which assaulted them in St. Louis. They manage to escape before the ship falls down into the sea. At Blue Raven Ridge, Candice Lee reports the event to General Hale.
The S.H.I.E.L.D. team returns to the Lighthouse. Shaw goes to see Rodriguez and Simmons to inform them of their return and overhears a conversation in which Simmons pronounces a sentence Shaw's mother used to tell him. Thus, Shaw realizes that Simmons and Fitz are his grandparents, although he keeps the information for himself while further working with Fitz.
Meanwhile, Werner von Strucker, now going under the alias of Alexander Braun, has a therapy session with Dr Levi Van Kempen. He reveals that he knows and remembers many details of Van Kempen's personal life that he never disclosed. As Van Kempen is stunned, von Strucker stabs him in the hand with a pen before being taken away by guards and sedated.
Von Strucker wakes up and happens to be in a secret base. He encounters Ruby Hale in the cafeteria but she ignores him. He finds her again in the physical training room and tells her that he remembers her, as they met in their respective childhoods. Suddenly, von Strucker takes Ruby hostage, prompting General Hale to join.
Hale explains that she has great plans for von Strucker and Ruby but that she needs information which is in von Strucker's mind. He refuses to help her and returns to his room. After he experiences a nightmare, he is visited by Ruby, who reveals that Hale wants her to gain his trust and is putting together a team. However, Ruby tells von Strucker that they do not need Hale and they can build their own team.
The day after, von Strucker agrees to team up with Ruby, who takes him to see Carl Creel. Hale asks Ruby how she convinced von Strucker to stay with them, to which Ruby replies that she simply told him the truth.
- Clark Gregg as Phil Coulson
- Ming-Na Wen as Melinda May
- Chloe Bennet as Daisy Johnson/Quake
- Iain De Caestecker as Leo Fitz
- Elizabeth Henstridge as Jemma Simmons
- Henry Simmons as Alphonso Mackenzie
- Natalia Cordova-Buckley as Yo-Yo Rodriguez
- Jeff Ward as Deke Shaw
- Catherine Dent as General Hale
- Dove Cameron as Ruby Hale
- Spencer Treat Clark as Werner von Strucker
- Jake Busey as Tony Caine
- Brian Patrick Wade as Carl Creel
- Paul Schackman as Doctor Levi Van Kempen
- Mark Fite as Doctor Joseph Getty
- Katie Amanda Keane as Alya Shaw (Recreation)
- Max Osinski as Agent James Davis
- Shontae Saldana as Candice Lee
- Danvers, Massachusetts
- Levi Van Kempen's Office
- Lake Ontario, Canada
- HYDRA Preparatory Academy
- Baton Rouge, Louisiana
- Fear Dimension (mentioned)
- S.H.I.E.L.D. Academy (mentioned)
- Maveth (indirectly mentioned)
- Hoover Dam (mentioned)
- Phil Coulson's Prosthetic Hand
- Sleeper Mechs
- Gravity Puck
- Time Di'Alla
- Kree Battle Axe (Recreation)
- Theta Brain-Wave Frequency Machine (mentioned)
- Cybertek Prosthetic Leg (mentioned)
- Thorazine (mentioned)
- Zima (mentioned)
- Operation Paperclip
- Wolfgang von Strucker
- Grant Ward
- April Van Kempen
- Levi Van Kempen's Wife
- Ian Quinn
- Alya Shaw
- John Garrett
- Project Deathlok
- John Knight (photo)
- Alexa Borel (photo)
- Candace Daniels (photo)
- Benjamin Froyd (photo)
- Rachid Zarrak (photo)
- Catherine Hasavari (photo)
- Rachel Antone (photo)
- Melanie Bradfield (photo)
- Bill Brummond (photo)
- Albert Einstein
- MC Hammer
- Flying Monkeys
- The episode and titular ship may take their name from Isaac Newton’s Philosophiæ Naturalis Principia Mathematica, a work which proposed the law of universal gravitation.
- When Alphonso Mackenzie carefully takes the Gravitonium, Phil Coulson teases him about his taste for MC Hammer's music by referring to the song U Can't Touch This.