Project T.A.H.I.T.I. (Terrestrialized Alien Host Integrative Tissue I) was a S.H.I.E.L.D. Level 9 project ordered by Director Nick Fury and overseen by Agent Phil Coulson. It involved the harvesting of an alien corpse for medicinal purposes. The tangible results were only to be used if an Avenger had died.
Since Nick Fury considered Agent Coulson just as important, he put the project in action after Coulson was killed by Loki during the Attack on the Helicarrier.
Project T.A.H.I.T.I. started in 2009, with the goal of creating a medicine that could save a mortally wounded Avenger. The head of the project was Agent Phil Coulson.
Project T.A.H.I.T.I. was located within the Guest House, guarded by Alpha and Beta. Inside the Guest House was a room labelled T.A.H.I.T.I. that contained a decaying Kree corpse being drained of its bodily fluids into multiple vials.
Each vial was labelled with different numbers for several serums but all with GH as the prefix, including GH.211, GH.289, GH.317, GH.319, GH.324 and GH.325. GH.325 has the effect of completely regenerating cellular tissue and implanting fragments of cellular memory of the host to the receiver.
Six S.H.I.E.L.D. agents with terminal illnesses were chosen by Phil Coulson to be participants in the project. After each was injected with GH.325, their ailments ceased. However, before Coulson could sign-off their release, the patients began showing signs of insanity which included writing in symbols.
Coulson conferred with Doctor Goodman, suggesting ending the program and destroying the alien specimen. Goodman adamantly disagreed. She suggested that the genetic memory of the alien was implanting itself onto the patients, so the Memory Overwriting Machine could be used to scrub the brains and give the patients new lives.
Coulson reluctantly agreed. The sixth patient, Sebastian Derik, feigned health and told Coulson that he did not want his memories erased. Coulson knew that he was carving the symbols into himself and gave him the procedure anyway. Phil Coulson regretted using the machine when he heard the screams that each person gave as they were kept conscious, with their skull caps removed, as their memories were changed.
Each person was released with no memory of the previous life as agents of S.H.I.E.L.D. and their personal backgrounds were documented with the deaths of their pasts. Rebecca Stevens became Janice Robbins and had the history to match. The name of "Rebecca Stevens" was written to have died of cancer.
However, the side effects on test subjects were too extreme, and Coulson resigned from his position and recommended the project's shutdown. However, that didn't stop the project.
When Nick Fury decided to resurrect Coulson after the Battle of New York, he allowed Project T.A.H.I.T.I. to do it. As part of the recommended process, Coulson's memory of his resurrection was wiped. In addition, his memories of leading Project T.A.H.I.T.I. were also wiped. Fury then had Coulson's recommendation tape buried inside his "grave".