In "Toxic," Abby is called on by the Army to work on a top secret research project, leaving the Gibbs team do investigate a murder with a forensic specialist. It doesn't take long before Gibbs's team discovers that the Army project Abby is supposed to work on is connected to their murder investigation. To boot, Abby was requisitioned by the Army to replace Phillip Heller, a scientist who had gone missing. They learn that the murder victim, marine private Marco Riggs, was initially hired to kill Heller - only Heller had fought back and accidentally killed Riggs first, with a stun gun.
When the team finds Heller down, they discover that the Army project that first he and now Abby were working on is a DNA project that could help sick soldiers AND be used effectively as a weapon. It is uncovered that a disgruntled bio-weapons specialist had duped the Army and set up the whole project to covertly create a new bidder than he would then sell to the highest bidder.