Dr. Who and the Daleks

Dr. Who (Cushing) and his granddaughters, Susan (Tovey) and Barbara (Linden), show Barbara's boyfriend Ian (Castle) the Doctor's latest invention, a time machine called TARDIS. When Ian accidentally activates the machine it transports them to a petrified jungle on a world devastated by an ancient nuclear war, fought between the Daleks and the Thals. (Although the planet is not named in the film, in its sequel it is retroactively revealed to be called Skaro, matching the name given in the television series.) At the conclusion of the war the Daleks, heavily mutated by radiation, encased themselves in protective machines and retreated into their city. The humanoid Thals survived the fallout through the use of an anti-radiation drug and became a peaceful race of farmers. The Thals' crops have recently failed, however, and they have journeyed to the petrified jungle to seek help from their former enemies. The Daleks, while determined to become the dominant race on the planet, are unable to leave the city due to their vulnerability to radiation and reliance on static electricity to power their travel machines.
Ian and Barbara are unnerved by the jungle and demand to return to London but the Doctor, eager to investigate the city, fakes a leak in one of the vital TARDIS fluid links to keep them on the planet. The group decide to search the city for the mercury needed to refill the link, stumbling across a case of Thal drug vials as they leave TARDIS. In the city the Doctor, on reading a Geiger counter, realises that the planet is radioactive and in view of the fact that they are feeling unwell, deduces that they are developing radiation sickness. Suddenly, the Daleks appear and capture the travellers, confining them to a cell and seizing the Doctor's fluid link for inspection.
The Daleks know of the Thal drug and want to reproduce it in large quantities so that they can leave the city and exterminate the Thals. They offer to let the humans use some of the drug to cure their sickness if the vials left outside TARDIS are brought to the city. Whilst carrying out the task Susan encounters Alydon, the Thal leader who left the vials. Alydon gives Susan a secondary drug supply to use in case the Daleks deviate from their promise and also lends her his plastic cape.
Upon Susan's return to the city the Daleks discover her secret drug supply but allow the humans to treat themselves with it. They then summon Susan to write a letter to the Thals, informing them that they wish to end post-war hostilities and will leave food in their control room as an act of friendship. The adventurers discover that when the Thals arrive, however, they will be ambushed and exterminated.
The group realises how the Daleks know everything they are saying and destroy the camera hidden inside the cell. They also plot a plan to escape the city. When a Dalek comes to the cell to deliver food and water, the Doctor and his companions splatter a chocolate like substance onto its eyestalk impairing its vision and eventually immobilise it by forcing it onto Susan's cape, thus insulating it from the electrically charged metal floor. Ian takes the place of the creature inside the casing and notifies another Dalek that he is taking the Doctor, Barbara and Susan to the control room for questioning. Now free, the travellers shout a warning to the Thals who are entering the city and escape with them into the jungle, but not before an elderly man, Temmosus, is killed by the Daleks.
Later the Daleks test the Thal drug on a number of themselves but find that it causes disastrous side effects. With no way of leaving the city, they decide to detonate a neutron bomb to increase the radiation on Skaro to a level which not even the Thals can survive.
At the Thal camp the Doctor urges Alydon to fight the Daleks to secure a safe future for his species. Alydon insists that the Thals are pacifist, but the Doctor tests this claim by ordering Ian to take Dyoni, Alydon's love, to the Daleks in exchange for the confiscated fluid link. Alydon punches Ian to the ground, showing the Thals that they can fight for things they care about. Alydon, Susan and the Doctor lead the tribe to the front entrance of the city, where they attempt to confuse the enemy's scanners by reflecting light off small mirrors to give the impression of greater numbers. The plan fails when the Daleks appear and the Thals scatter, unable to prevent Susan and the Doctor from being captured.
Meanwhile, Ian and Barbara, guided by the Thals Ganatus, Antodus and Elyon, set out to infiltrate the city from the rear. While navigating a swamp Elyon is killed by a marsh-dwelling mutation and the party is eventually forced to jump a chasm to proceed any further. Antodus falls short and plunges into the void, but manages to cling to the uneven rock face and is pulled up by the others.
In the city control room the Daleks ignore the Doctor's appeals as they start the bomb countdown. Ian, Barbara, Ganatus and Antodus penetrate the city and join Alydon and the rest of the Thals, who have returned determined to rescue Susan and the Doctor. The Thals and humans enter the control room and struggle with the Daleks while the Doctor yells for someone to stop the bomb detonation. Ian attracts the Daleks' attention and dives for cover as they fire at him in unison. The Daleks inadvertently destroy their own control console, disabling themselves and freezing the countdown. The Doctor then retrieves the TARDIS fluid link.
In the jungle, the Thals bid farewell to the Doctor and his companions and express their gratitude with special gifts. When the travellers depart in TARDIS they materialise not in London, however, but on a battlefield in front of an advancing Roman army.

Question: Why does Alydon give Susan a second drug supply?
Answer: In case the Daleks deviate from their promise.