Contains spoilers for this episode and earlier episodes.
The returning author, Stanford Pines, greets his brother Stanley with anger rather than thanks. Stan explains to the twins that the name he has used all summer originally belonged to Ford. As boys, the brothers planned to sail the world together, but Ford's scientific ability offered him a different future. Stanley accidentally damaged the experiment that might have won Ford a place at his dream university. Their father threw Stanley out, and the brothers spent years apart. Ford eventually moved to Gravity Falls to investigate its anomalies, filling the journals and working with Fiddleford McGucket on a portal. When the project became dangerous, he summoned Stanley for help.
Ford wanted Stanley to take the remaining journal far away, but Stanley heard another rejection. Their argument became a fight that accidentally activated the portal and pulled Ford through it. Stanley stayed behind, took his brother's identity and turned the house into a tourist attraction to fund his search for a way to bring him back. Decades of secret work explain the lies, false identities and hidden laboratory. In the present, Ford helps erase the agents' memories of their investigation and sends them away with a cover story. The immediate threat passes, yet the brothers remain divided over the house and the children. Mabel worries that she and Dipper could also grow apart, and he reassures her before they sleep.