Growing up in Vault 101, Isaac had his entire life planned out for him: predictable, unremarkable and, above all, safe. However, when his father disappeared from Vault under mysterious circumstances, he set in motion a series of events which ended with the Vault Overseer dead and Isaac banished to the ruins of Washington D.C. Isaac sought shelter in a nearby settlement, Megaton, but the post-apocalyptic wasteland was a far cry from the controlled environment he'd grown up in. His innocence and eagerness to ingratiate himself with the locals almost proved Isaac's downfall on several occasions, as he took on tasks that other, more experienced mercenaries had wisely avoided.
Tales of the melancholic "Lone Wanderer" soon spread throughout the Capital Wasteland, with some claiming that the teenaged outcast was capable of performing impossible miracles. Alongside the Brotherhood, Isaac helped reclaim the National Mall; he single-handedly ensured the downfall of the wasteland's largest slave-trading ring at Paradise Falls, and allegedly even prevented a nuclear explosion in the town of Megaton, all whilst searching for his missing father. As word of his deeds spread, it became increasingly difficult to separate fact from the fiction - and the boy from the hero.
The people he met out in the wasteland seemed convinced enough, citing a kind of big-hearted grit which could apparently move even the most stubborn of mountains, but Isaac's many detractors noted that his glowing reputation could only reflect warmly on his new allies in the Brotherhood of Steel, bolstering their attempts to take control of the East Coast.
Isaac was eventually reunited with his father inside the Jefferson Memorial, where James Adams Sr. met his end at the hands of the villainous Enclave. In doing so, he managed to protect his life's work on "Project Purity": an audacious plan turn the Potomac River into clean, potable water, with the help of a pre-war terraforming device.
In the wake of his father's death, Isaac went into near-total seclusion in the fortified settlement of Rivet City. He was eventually convinced by the Brotherhood to recover the G.E.C.K. from Vault 87 before the Enclave could lay their hands on it, resulting in a brief-but-harrowing captivity at the Enclave's base of operations in Raven Rock. However, when the time came to install the G.E.C.K. inside the Jefferson Memorial, Isaac discovered that somebody would have to enter the same irradiated chamber which had killed his father - sacrificing their life to see the Project completed. As he steeled himself to die, Isaac's friend and mentor, Brotherhood Sentinel Sarah Lyons, chose to take his place instead, locking herself in the chamber before he could intervene.
Isaac's apparent role in Lyons' death made him instant persona non grata amongst the Brotherhood, and earned him the lasting enmity of its future leader, the young Arthur Maxson. Isaac briefly sought sanctuary in Rivet City, but it soon became apparent that his mere presence would endanger anybody close to him. Thankfully, Rivet City's ex-Courser turned security chief, Harkness, was able to put Isaac in contact with the Railroad. Using the same routes they had set up for rescuing Institute synths, the Railroad were able to smuggle Isaac out of the wasteland without the Brotherhood's knowledge.
Since then, Isaac has has been operating as a Railroad agent under the codename "Wanderer". Utilizing his talent for field work, the Railroad put Isaac to use across the continental US, extracting synths from so-called "hot zones" under Institute surveillance. By 2287, Isaac was working with the Railroad chapter in Massachusetts, ostensibly to assist them with the vital work of evacuating and re-housing the sudden outpouring of synth refugees in the region.
Though suspicion about his true motives abounds, Isaac was able to keep his cover story mostly-intact from even the Railroad's high-ranking agents, with only a select few in the organization even aware of his true identity. However, his legendary luck was about to run dry. When the Brotherhood's mobile command center appeared in the skies above Boston, its troops on the ground were suddenly put on the lookout for a man matching Isaac's description. Aware that his cover had been blown, Isaac nonetheless insisted that he was about to hit a major breakthrough in his research, perhaps even uncovering the location of the Institute itself - something long thought to be out of the Railroad's reach. As the net closes in on him from all angles, a world-weary Isaac is prepared to risk everything to ensure the Railroad's success. However, even he couldn't foresee the potential consequences his actions would bring down upon them.
"Wanderer" in the New England Commonwealth, circa 2287. Note the Power-Fist on his right arm.
Isaac aged approx. 18 years old. This is how he would've appeared during the events of Fallout 3.
Isaac aged approx. 28 years old. Nearly a decade of wasteland living has hollowed him out a bit, and he's picked up a few new scars.
A note on Ships: Isaac/Harkness is the OTP, but Isaac/Everyone is the endgame. Regarding the obvious question of Isaac/Charon, their relationship was... complicated. Charon's contract precluded the possibility of informed consent between them for a long time, and also Isaac at eighteen years old was a mess. A beautiful, compelling mess, but a mess all the same. I think there was a degree of tenderness between them, but by the time they realized it after Project Purity they were already on different trajectories. I'm a big fan of Maxson having weird, fucked-up feelings about the adult figures in his life, so I can see a level of unrequited Isaac/Maxson. Isaac tends to be my little black dress as far as Fallout pairings are concerned - given that he feasibly appears in three of the four games (Fallout 3, Fallout: New Vegas and Fallout 4).