Stel was the chief investigator of the Security Directorate under Administrator V'Las in 2154. In that capacity, he was involved in the conspiracy to frame the Syrrannites for the bombing of the United Earth Embassy on Vulcan.
To that end, Stel planted a bomb with the DNA of T'Pau, a well-known Syrrannite, in order to implicate her. The resultant explosion resulted in the deaths of thirty-one Humans and twelve Vulcans. Although the plan to blame the Syrrannites for the bombing initially succeeded, a mind meld, conducted by Ambassador Soval with comatose victim Corporal Askwith, revealed Stel's involvement in the conspiracy.
In an attempt to preserve the plot, however, Stel was subsequently framed as a Syrrannite, Administrator V'Las claiming that relevant documents had been found in Stel's home. (ENT: "The Forge", "Awakening")